Chore: hardcode default scope

Problem: 
- it seems to be impossible to set a scope on context, that is also accessible in IEx console
- the list of known ancestors of IEx console does NOT include THE main application

Solution / Workaround: 
- we hardcode a default scope UUID ("000-000000...") and use re-scoping only in unit tests
- that seems to be an OK compromise

lib/context.ex

@@ -6,7 +6,7 @@ defmodule Essig.Context do
   end
 
   def current_scope do
-    ProcessTree.get(@appkey)
+    ProcessTree.get(@appkey, default: "00000000-0000-0000-0000-000000000001")
   end
 
   def assert_current_scope! do